WASHINGTON
— A highly respected US State Department official was arrested last week, suspected of passing secret government documents to Taiwanese intelligence agents. And earlier this month, word leaked that the FBI is investigating a Pentagon official for possibly providing classified information to Israel.

The cases are alarming enough, in that two men in sensitive positions may be betraying their country. But together they also highlight one less well-known fact: Espionage against the US is increasing, rather than decreasing,in the post-cold-war era, experts say. Because the US has become the sole dominant military and economic power in the world, friends and foes alike want access to more information than the US readily shares with them.

"There is an ever-present threat of foreign intelligence collection against the US," says a US law enforcement official. "And it's not only the traditional, like military capabilities. It's foreign policy planning, and there is a vast interest in patent materials, not only for machines, but for research."

The Federal Bureau of Investigation regularly updates a closely held list of the countries that threaten national security due to espionage operations. "The top five countries on that list are China, Israel, Russia, France, and North Korea. Others include Cuba, Pakistan, and India," says an official close to the FBI.

Of course, the US isn't above snooping on its friends and foes either. Just a couple of years ago, France deported two Americans accused of conducting espionage there.

Arthur Hulnick, a professor of international relations at Boston University, former CIA official, and author of a new book, "Keeping us Safe: Secret Intelligence and Homeland Security," says a student recently asked him, "Do we ever spy on our friends?"

"Only when we have to," Professor Hulnick responded, tongue in cheek. "Of course we do," he adds. "Why wouldn't we do it, if they don't give us what we want? And why wouldn't they do it to us?"

Government officials and outside experts say foreign agents focus on four primary areas: US military capabilities, foreign policy strategy, technological expertise, and business plans. The first two are the most common, according to the US law-enforcement official. But he says that foreign intelligence agents don't target just people who work at the Pentagon. They try to make inroads with contractors - those responsible for, say, a ship or airplane. Or subcontractors - those responsible for small parts that make up the larger ships, airplanes, and tanks.

The FBI carries out a number of sting operations in these areas. But many of them never become public. "They're just not prosecutable," the law enforcement agent says. "The persons involved are usually outside the jurisdiction of the court because they have diplomatic immunity."

In these cases, they are asked to leave the US and are prevented from reentering. Another problem, according to the law-enforcement official, is that when the cases involve highly classified military activity or industrial espionage, the government and private sector choose not to prosecute. "They don't want to exacerbate the situation by publicizing it, revealing trade secrets in litigation" he says.

The FBI says it has a large counterintelligence unit, but the numbers of agents involved and numbers of ongoing cases are classified. And FBI counterintelligence teams routinely watch employees of foreign embassies.

Still the recent arrest and leak of another possible infraction indicate progress is being made.

Donald Keyser, a career State Department employee and expert on US-Chinese-Taiwanese relations, was released on a $500,000 bond last week after he was officially charged with lying about an unsanctioned trip to Taiwan.

According to an affidavit filed in the US District Court in Alexandria, Va., FBI agents followed Mr. Keyser this summer and saw him pass documents to two Taiwanese agents. The court documents also said Mr. Keyser made an unsanctioned trip to Taiwan after official visits to China and Japan. He never reported the trip to Taiwan to his superiors, and allegedly later lied about it.

"If it indeed turns out to be true, it's a classic approach," says Hulnick. "The way you entice someone is to ask for documents. First you ask for something simple, like a phone book. It doesn't have to be secret. And little by little, you begin asking for the good stuff."

Hulnick goes on to say that it's totally understandable that Taiwan would want to know US aims toward China and Taiwan.

"We don't have relations with Taiwanese technically, so they want to make sure we're not selling them out to the People's Republic of China," he says. "The best way to do that is to get somebody on the inside who is willing to cooperate."

The other FBI investigation apparently resulted from a FBI routine surveillance of a senior Israeli diplomat. The bureau is now investigating a Pentagon employee and officials from an Israeli lobby for possibly handing a highly classified foreign-policy document about Iran to Israeli officials. That investigation has not yet resulted in charges, but officials say the scope of that probe has broadened.